1. Field of the Invention
The invention relates generally to optical communications, and more specifically to a variable attenuator for removing optical energy from a waveguide in a controllable manner.
2. Description of the Prior Art
The telecommunication industry is making increasing use of optical communication systems for high-bandwidth transmission of voice, video, and data signals. In optical communication systems, it is frequently necessary or desirable to precisely adjust optical signal levels entering various system components. Adjustment of optical signal levels is typically achieved by incorporating variable optical attenuators into the optical circuits. In one example, a variable optical attenuator may be employed to equalize power levels among separate channels of optical circuits implementing wavelength division multiplexing (WDM). Variable optical attenuators may also be employed to avoid exposing optical detectors to excessive signal levels, which may damage the detectors and cause them to become inoperative.
Various references in the prior art disclose attenuators for use in optical circuits. Examples of such attenuators include those described in U.S. Pat. No. 4,762,383 to Yamashita et al. (xe2x80x9cTwo Dimensional Light Beam Deflectors Utilizing Thermo-optical Effect and Method of Using Samexe2x80x9d); U.S. Pat No. 5,881,199 to Li (xe2x80x9cOptical Branching Device Integrated with Tunable Attenuators for System Gain/Loss Equalizationxe2x80x9d), and; U.S. Pat. No. 5,966,493 to Wagoner et al. (xe2x80x9cFiber Optic Attenuators and Attenuation Systemsxe2x80x9d). The attenuators described in the foregoing references, as well as other prior art attenuators, are known to suffer from operationally significant problems or limitations. These problems include sensitivity to ambient temperature, high power consumption, limited or no adjustability, the occurrence of cross-talk between adjacent channels, high coupling losses, bulkiness and slow responsiveness.
Thus, there is a need in the optical communications art for a variable optical attenuator which overcomes the problems associated with prior art devices.
In accordance with an embodiment of the invention, a variable optical attenuator is provided having at least one elongated core, a cladding surrounding the core, and a controllable thermal source and a heat sink arranged on opposite sides of the core and defining therebetween a first or vertical axis oriented transversely to the longitudinal axis of the core. The core and cladding collectively form a conventional waveguide structure, which normally confines optical energy propagating along the longitudinal axis of the core by virtue of the difference in refractive indices between the core and cladding. The core and cladding materials are preferably selected such that their thermo-optic coefficients (i.e., dn/dT, where n is the local refractive index and T is temperature) are closely matched within the ambient temperature range of interest. Matching the thermo-optic coefficients of the core and cladding ensures that the waveguide confinement (a function of the difference between the refractive indices of the core and cladding) is substantially invariant with respect to ambient temperature, thereby obviating the need to provide heating or cooling of the waveguide package.
When attenuation of the optical energy propagating along the core is desired, a control signal is applied to the thermal source, which in turn causes a temperature gradient to be developed along the first (vertical) axis extending between the thermal source and the heat sink. The temperature gradient results in a xe2x80x9ctiltedxe2x80x9d or asymmetric refractive index profile within the core wherein the refractive index of the core increases along the first axis from the proximal core-cladding boundary (the boundary nearer to the thermal source) to the distal core-cladding boundary (the boundary more remote from the thermal source). Extraction of optical energy from the waveguide occurs when the local refractive index at the higher-temperature areas of the core (those adjacent the proximal boundary) is depressed below that of the local refractive index of the cladding immediately adjacent to the distal core-cladding boundary. This condition causes at least a portion of the optical energy propagating along the core to be transversely deflected in the direction away from the thermal source (i.e., toward the heat sink). The amount of optical energy extracted from the waveguide is controlled by adjusting the signal (for example, a voltage) applied to the thermal source.
The invention further encompasses an attenuation system, incorporating a variable optical attenuator of the foregoing description, in which a control circuit applies a variable signal to the thermal source in accordance with a desired degree of channel attenuation and with feedback information obtained by monitoring the power level of optical energy traveling through the core. The attenuator and attenuation system of the present invention may be advantageously employed in any number of optical circuit applications where it is necessary or desirable to control optical power transmission within individual optical channels.